The production of electric power from various types of alternative energy sources such as for instance wind turbines, solar power plants and wave energy plants is not continuous. The production may be dependent on environmental parameters such as for instance wind speed (for wind turbines), sunshine intensity (for solar power plant) and wave height and direction (for wave energy plants). There is very often little or no correlation between energy production and energy demand.
One known approach to solve the problem of uncorrelated electric power production and electric power demand is to temporarily store energy, which has been produced but which has not been demanded, and to release the stored energy at times at which there is a high demand. In the past there have been suggested many different methods to temporarily store energy. Suggested methods are for instance (a) mechanical energy storage methods e.g. pumped hydro storage, compressed air storage and flywheels, (b) chemical energy storage methods e.g. electrochemical batteries and organic molecular storage, (c) magnetic energy storage, and (d) thermal energy storage.
The document US 2010/0301614 A1 discloses an installation for storing and returning electrical energy. The disclosed installation comprises a first enclosure and a second enclosure each containing a gas and porous refractory materials suitable for transferring heat by contact between said porous refractory materials and a gas flowing through the respective enclosure. The disclosed installation further comprises a compressor and an expander for the gas flowing in pipes between each of the ends of an enclosure connected to an end of the other enclosure. The porous refractory materials may be formed as bricks such as fire clay or a similar material.
The document WO 2009/044139 A2 discloses an apparatus for storing energy. The disclosed apparatus comprises (a) a compression chamber means for receiving a gas; (b) a compression piston means for compressing gas contained in the compression chamber means; (c) a first heat storage means for receiving and storing thermal energy from gas compressed by the compression piston means; (d) an expansion chamber means for receiving gas after exposure to the first heat storage means; (e) an expansion piston means for expanding gas received in the expansion chamber means; and (f) a second heat storage means for transferring thermal energy to the gas expanded by the expansion piston means. The cycle used by the apparatus has two different stages that can be split into separate devices or can be combined into one device. The first heat storage means and the second heat storage means may be equipped with gravel or rocks representing a heat storage material.
However, at many locations where alternative energy sources are erected and where the energy production does not correspond to the energy demand gravel or rocks are not available in big amounts. Often, e.g. in Denmark, the landscape is characterized by sediments and deposits like sand from a number of ice ages. The sedimentary rocks like flint stone being available in such regions are not suited for heating and repeating thermal cycles because of the risk of stone fracturing. The preferred rocks for high temperature storage is called igneous rocks like granite but this type of rock is not available everywhere and needs to be transported which is costly.
There may be a need for providing a thermal energy storage and recovery device being suitable for using a widely available and cheap material as a heat storage material.